royal icing takes color a lot better than buttercream does (there is just a lot of "fat" in buttercream that is harder to color). So it doesn't take as much black coloring. I have used wilton black or Americolor super black with no problems.

I use wilton Black to get the icing dark...then I use Americolor Super black to get it black, black. I find that if I use all Americolor, the icing tends to have a bitter taste so I started using both and I don't get the bitter after taste anymore.